The conventional printing systems requiring fountain solutions have the serious problem that ink is emulsified or the fountain solutions are contaminated with ink because of difficulty of delicately controlling a balance between the fountain solutions and ink, resulting in poor ink density or generation of background scum to cause broke. In contrast, the waterless printing plates have many advantages because no fountain solutions are required. As to such waterless printing plates for conducting lithographic printing without use of the fountain solutions, various kinds of plates are proposed, for example, in JP-B-44-23042 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-46-16044, JP-B-54-26923, JP-B-56-14976, JP-B-56-23150, JP-B-61-54222, JP-A-58-215411 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-2-16561 and JP-A-2-236550. In particular, a waterless printing plate has very excellent characteristics, which comprises a support having provided thereon a primer layer, a photopolymerizable light-heat conversion layer and a silicone rubber layer in this order, wherein exposure polymerizes and hardens the photopolymerizable light-heat conversion layer to strengthen adhesion of the exposed area of the light-heat conversion layer to the silicone rubber layer, and development removes only the unexposed area of the silicone rubber layer, thereby forming an image area.
On the other hand, with the recent rigid progress in prepress systems and output systems such as image setters and laser printers, methods for obtaining printing plates by converting printing images to digital data, and using new prepress processing systems such as computer-to-plate and computer-to-cylinder systems have been proposed. Accordingly, new types of printing materials for these printing systems have been desired, and the development thereof is proceeding. However, although many conventional lithographic printing plates using fountain solutions have been provided for these techniques, waterless printing plates are scarcely known.
Formation of waterless printing plates by laser beam writing is described, for example, in JB-B-42-21879, JP-A-50-158405, JP-A-6-55723, JP-A-6-186750, U.S. Pat. No. 5,353,705 and PCT International Publication No. WO-9401280. These references describe that an ink-repellent silicone rubber layer is provided on a light-heat conversion layer containing a laser beam absorber such as carbon black and a self-oxidizing binder such as nitrocellulose, the ink-repellent silicone layer is partially removed by laser beam irradiation to form an ink-receiving area, thereby conducting waterless printing. However, in these methods, the removal of the silicone rubber layer depends on ablation of the light-heat conversion layer by laser beam irradiation, so that the linearity of thin lines and the circularity of halftone dots become poor, resulting in unsatisfactory printing images. Accordingly, an improvement thereof has been strongly desired. Further, they have the disadvantage fatal to the printing plates that they are easily scratched in treating the printing plates and in printing because of weak adhesion between the light-heat conversion layers and the silicone rubber layers, and ink adheres to the resulting scratches to form image areas. Some references disclose addition of a silane coupling agent to the silicone rubber layer to compensate for this disadvantage. However, it is insufficient to enhance the adhesion between the light-heat conversion layer and the silicone rubber layer, and the scratch resistance is little improved.